Bacteria have been examined that have developed very efficient and different mechanisms for tolerating heavy metals. Often normal toxic levels of metals have no effect on cell growth of resistant strains. In many organisms, the genes controlling metal resistance are carried on plasmids, which provide the bacteria with a competitive advantage over other organisms when metals are present. Not all metal-resistant bacteria contain plasmids. For example, a Pseudomonas sp. (research at the Canada Centre for Inland Waters and the University of Guelph) resistant to 1000 #g/ml lead, also displays multiple antibiotic resistance. Vertical and horizontal agarose gel electrophoresis of cleared lysates revealed that plasmids were not present, even though the organism was tolerant to extremely high concentrations of lead.Plasmid-encoded resistance may provide organisms with efflux and bypass mechanisms, enzymes which catalyze the transformation of metals to volatile forms, or make the bacterial cell wall impermeable to the metal(s).A problem still remains in actually defining the concentrations that distinguish metal-resistant from metal-sensitive bacteria. Some researchers have proposed mathematical equations, and used statistical an.alysis to solve this problem. Nevertheless, standard concentrations have not been uni-versally proposed and/or accepted by the scientific community. This task is complicated by the various forms of metals used, the effect of media components, pH, and culture conditions, which have the capability of influencing the toxicity of the metal.Research on metal-bacteria interactions is in some ways still in its early stages of development. Nevertheless, significant advances in scientific knowledge have been achieved over the last 20 years. To fill the existing gaps in knowledge, research will need to be conducted on the extent of metal resistance in bacteria, the relationship between metal and antibiotic resistance, mechanisms specified by the plasmids, and the ecology, physiology, and genetics of gene transfer in the natural environment.Since some microorganisms are responsible for environmental metal transformations, they may also serve as bioassay indicator organisms in polluted and non-polluted environments. Moreover, there may be potential biotechnological applications for metal-resistant bacteria in the area of toxic metal control in waste-water treatment. INTRODUCTIONThe genetic basis for metal resistance in bacteria is an area of intensive research both in the en-0168-6445/85/$03.30

R-plasmid transfer in Escherichia coli was investigated in nutrient broth, sterile soil, and sterile stream water. Plasmid transfer occurred in broth cultures at 30 and 22 degrees C, but not at 15 degrees C. R-plasmid transfer was not observed at 30, 22, and 15 degrees C in nonamended sterile soil and stream water. The addition of nutrients to sterile stream water and soil allowed plasmid transfer to occur at temperatures ranging from 15 to 30 degrees C. R-plasmid transfer was also observed in soil adjusted from 20 to 100% of its water-holding capacity.